Immune tolerance, the phenomenon by which the allograft is accepted without immunosuppression while preserving the recipient's protective immunity, represents a solution to the problems of acute and chronic rejection and the resulting long-term reliance on toxic immunosuppressive therapies. The significant success of transplantation tolerance studies in rodent models has suggested that similar tolerance-induction techniques involving bone marrow transplant and hematopoietic chimerism could be achieved in preclinical and clinical situations, thus revolutionizing solid organ transplantation. Non-human primate models have a number of important attributes that allow them to serve as critical preclinical models in order to bridge the basic insights gained in mice and the application of these insights to patient care. Among the most prominent of the tolerance induction strategies are CD28/CD40 T cell costimulation blockade and mixed chimerism induction. By taking advantage of our ability to induce chimerism using mobilized peripheral blood stem cells from living Rhesus macaque donors, we propose to perform a systematic analysis of impact of a costimulation blockade and chimerism-based tolerance induction strategy in transplant pairs having varying degrees of MHC disparity. These studies also are focused on understanding the immune consequences of transplant, specifically on evaluating the anti-donor response and the preservation of protective immunity in the peritransplant period. The unifying purpose of our proposal is to develop clinically applicable protocols for the induction of tolerance to solid organ allografts while preserving immune competence in the transplant recipient. Specifically, the aims in this project will address 1) the effectiveness of a CD28/CD40 costimulation-blockade-based chimerism/tolerance induction protocol on transplants displaying varying degrees of MHC matching between the donor and recipient, 2) the necessary components of the immunomodulatory strategy for chimerism and tolerance induction, and 3) the efficacy of inhibiting Natural Killer cell-mediated alloreactivity in order to decrease the need for recipient conditioning and/or donor peripheral blood stem cells to promote tolerance across MHC barriers. We believe the ability to induce stable donor chimerism and immune tolerance in this transplant setting would have a large impact on the outcome of transplantation, and holds the promise of relieving many transplant recipients from the requirement for complicated life-long immunosuppressive regimens and their attendant toxicities.